This invention relates generally to wet forming processes for making fluff pulp from soften wood pulps and, more particularly, to improved processes for making fluff pulp sheets which eliminate many of the unwanted fiber-to-fiber bonding (fiber bundles) that may be contained in the sheet to produce consistent and uniform quality fluff pulp. These improved processes also permit the manufacturer to control the consistency of the stock being formed by localized dilution to achieve a better cross-machine directional basis weight allowing the manufacturer to produce high quality fluff pulp while using low headbox consistency. Fluff pulp produced by the processes of the present invention is soft, flexible, and has a lower content of knots or hard spots. The processes of the present invention are capable of producing fluff pulp sheets having low variability in weight, moisture, Mullen strength and other physical sheet attributes. Accordingly, a fluff pulp sheet made in accordance with the present invention should have low shred energy while possessing high shred quality which results in significantly reduced fiberization energy when the sheets are ultimately processed. The invention is especially useful for the production of fluff pulp intended for use as the absorbent layer in disposable diapers, sanitary napkins, absorbent hygienic products and airlaid products.
Absorbent products employing fiberized wood pulp have been available for many years. This basic wood pulp used in such products is usually termed “fluff pulp.” In the United States, fluff pulp is most typically made from a fully bleached southern pine kraft process pulp produced in relatively heavy caliper, high basis weight sheets. The product is rewound into continuous rolls for shipment to the customer. Since the roll product is intended to be later reprocessed into individual fibers, low sheet strength is desirable and typically little or no refining is used prior to roll manufacturing. The requirements for surface uniformity and formation are similarly moderate.
At the customer's plant, the rolls are continuously fed into a device, such as a hammermill, to be reduced as much as reasonably possible to individual fibers. Defibration is the process of freeing the fibers from each other before the fluff pulp enters the product forming machinery. The fiberized product is generally termed a cellulose “fluff.” For example, the fluff pulp can then be continuously air laid into pads for inclusion in the intended product. The most demanding application of fluff pulps is in producing air-laid products, used, for example, in serving utensils and various towel applications in homes, industry and hospitals. As is mentioned above, fluff pulp sheets for air-laid products are usually defiberized in a hammermill. Fluff pulp sheets, however, may contain significant numbers of fiber bundles which are bonded together during the sheeting process. These unwanted fiber bundles, often referred to as knots, nits, bones and flock in the industry, present a problem during defibration. The hammermills used for fluff production are very large energy consumers and fiber bundles present in the fluff pulp sheets will increase the amount of energy expended during defibration. Also, while vigorous defiberizing can reduce the knot content, it is at the expense of considerable fiber breakage and a high resulting content of very fine dusty material. To offset this problem, the pulp mill may need to add chemical debonders prior to sheet formation. Therefore, important parameters that are considered for dry defibration are shredding energy, i.e., the amount of energy needed to shred the sheet and knot content, i.e., the amount of clumps of fibers bonded to each. In heavy manufacturing operations, reduction in energy consumption will ultimately lead to less costly products. Moreover, many manufacturers require high quality fluff pulp to be used in their products due to customer demands. Accordingly, manufacturers of fluff pulp sheets are concerned in creating sheets having low shredding energy while still providing high quality fluff. Lower quality fluff pulp sheets cannot be used in certain applications and as such are often discounted for use in manufacturing lower quality products.
Wood pulp softness can be expressed in terms of properties such as Mullen strength (the strength of pulp or a pulp product, measured in kilopascals (kPa)), and Kamas energy (the energy required to convert a given amount of pulp or pulp product to a fluff material, measured in watt hours per kilogram (Wh/kg)). Mullen strength can be thought of as the energy required to pop a hole in the sheet. Some in the industry refer to this energy as “burst energy.” Mullen strength is a good indicator (but not full proof) of the energy needed to shred the sheet (shred energy). Typically, the lower the Mullen strength, the easier it is to shred the fluff pulp sheet. Lower values of Mullen strength and Kamas energy also correlate to softer, increasingly debonded, pulp. While it is desirable to the manufacturer to decrease Mullen strength, it should not be done at the expense of shred quality.
In the art of making fine paper, stock is usually ejected from a device known in the industry as a headbox so as to land gently on the moving fabric loop, known as a forming wire, which moves at a speed typically between plus or minus 3% of the wire speed, called rush and drag respectively. In the manufacture of fluff pulp, the equipment is usually run at about +10% rush. Excessive j/w ratio helps the Mullen strength. Water drains from the stock through the forming wire so that a web is formed on the forming wire. Excessive rush or drag can cause more orientation of fibers of the web in the machine direction and can give differing and sometimes unwanted physical properties in machine and cross directions. Manufacturers, therefore, are concerned about fiber orientation and accordingly have to control the orientation of fibers being deposited on the forming wire in order to achieve the desired physical properties.
As was mentioned above, wood fibers have a tendency to attract to one another, forming clumps, the effect being called flocculation. Flocculation is lessened by lowering consistency and or by agitating the slurry entering or in the headbox. However, defloccullation becomes very difficult at much above 0.5% consistency. Minimizing the degree of flocculation is important to the physical properties of the fine paper or fluff pulp.
Usually, the stock is supplied at extremely high pressure to the headbox by means of pumping equipment and the stock is ejected from the headbox through a device known as slice lip. Accordingly, it is essential that the rate of flow of stock through a distributor tube disposed at one side of the headbox be the same as the rate of flow of stock moving through a distributor tube disposed at the opposite side of the headbox. The rate of flow of stock is usually defined as the number of cubic feet of the stock passing a particular point every minute. It is necessary that the rate of stock flow remain constant or as constant as possible throughout the headbox. The amount of fiber per unit area (basis weight) of the formed web should be ideally constant across the width of the machine and along the machine direction. If the stock has been thoroughly mixed and if the slice lip opening is the same along the entire cross-machine directional width of the headbox, then the weight of the fibers within the stock per inch of width across the ribbon of stock ejected through the slice lip should be substantially constant. The resulting web should then have a uniform basis weight in a cross-machine direction. However, in practice, it is often difficult to maintain a constant stock supply pressure and a uniform consistent in the stock. Accordingly, maintaining an even distribution of fibers within the stock present problems when endeavoring to maintain a uniform basis weight across the width of a formed web.
The manufacturers of fluff pulp also face the problem of maintaining a controlled cross-machine directional basis weight of the formed web. Manufacturers must control the basis weight of the formed web to improve the quality of the end product. Accordingly, the fluff pulp manufacturer must control the basis weight without compromising fiber orientation profile. Additionally, the manufacturer must also be mindful of the need to simultaneously minimize the degree of flocculation in order to attain the desired physical properties of the fluff pulp.
Accordingly, it would be desirable to provide processes for forming fluff pulp sheets having improved bulk, softness and reduced inter-fiber bonding without sacrificing the absorbent properties of the pulp. Also, there has been a need for processes for producing high quality fluff pulp sheets that have significantly lower Mullen strength (burst energy) without losing shred quality. There is also a need to achieve a more uniform basis weight profile without compromising the fiber orientation profile. An improved and more uniform cross-directional weight basis can promote more stable operation in the hammermill and uniform final user product. The novel processes of the present invention fill these and other needs.